So, first with vlookup():

A formula so you can copy & paste:
VLOOKUP(D2,$A$1:$B$5,2,1)
For those who think vlookup() is a poor choice, here is the use of index() with match(), which needs more typing and no benefit in this situation:
INDEX($B$1:$B$5,MATCH(D2,$A$1:$A$5,1))